Electron microscopy is used to detect, measure, and analyze features and constituents present in very small areas of material specimens and samples (the terms specimen and sample are used interchangeably). These specimens and samples are placed into the analytical instrument and interact with the electron beam of the electron microscope. These specimens introduced into the analytical instrument have contaminants and other foreign species adsorbed, attached or present on the sample. These contaminant species, upon interaction with the incident electron beam, distort the analytical measurements and detract from the accuracy and quality of the analytical measurements. Most often, the electron beam reacts with the impurity species, forming deposits on the surface of the sample. These deposits in turn interfere with secondary electron emission, X-ray emission or electron image formation in the analytical instrument. Whenever these impurities are present in the analysis chamber, these deposits are produced. The contaminant deposits when present show a demonstrated reduction in contrast and resolution in both transmission electron microscopes (TEM) and scanning electron microscopes (SEM).
All modern transmission electron microscopes and some scanning electron microscopes utilize sample introduction and manipulation stages that extend into and between the electron beam lens pole pieces. This is known as “in the lens” imaging. A sample mounting platform is placed at the end of a long, hollow cylindrical “stage” rod. The sample stage extends through the instrument airlock and into the microscope analysis chamber. The length of this rod enables both accurate positioning of the sample at the object plane of the electron microscope and accurate adjustment of the sample, once placed at the correct location. The electron beam is precisely focused on an aperture in the center of the analysis chamber. The sample is located precisely below this aperture. After passing through the aperture and sample, the electrons are collected for subsequent analysis. Adjustment of the sample orientation with respect to the electron beam is achieved with a goniometer connected to the stage mechanism. The goniometer is an angular adjustment mechanism that allows precise positioning of the sample with respect to the electron beam.
Samples and specimens for transmission electron microscopy must undergo preparation before they can be analyzed in the instrument. The samples are thinned using various preparation techniques such that the electron beam can pass through the sample, and the TEM imaging optics can properly form a virtual image at the microscope image plane. The electron beam is very tightly focused within this region of the instrument. Electron beam properties at the instrument object plane place great constraints on the sample size and thickness.
The electron microscope sample introduction and manipulator stage utilizes a vacuum-airlock mechanism to ensure that the samples are introduced into the analysis chamber with no disruption to the vacuum in the electron microscope. The vacuum airlock mechanism is implemented with multiple O-ring seals positioned at intervals along the hollow stage support rod. The system also uses a location pin or fixture that has a precise relationship to these O-ring seals. Typical operation of the airlock is as follows: Using the stage support rod, the stage is inserted into the airlock until the locating pin reaches a position stop. When the sample stage reaches this stop, it is rotated through a large angle, typically 90 degrees or greater. Rotation of the stage causes the location pin to activate the airlock pump-down cycle. Once an appropriate pressure level is reached in the airlock, an isolation valve between the airlock and the analysis chamber is opened and the sample stage passes into the microscope analysis chamber. This system ensures that sample transfer occurs at the correct time in the evacuation sequence and that the sample is positioned reliably and reproducibly in the microscope analysis chamber.
Electron microscopy systems, especially TEM systems, are highly susceptible to chemical contaminants. These contaminants can be introduced into the microscope system by four mechanisms: specimen contamination, stage contamination, airlock contamination, and leaks. These contaminants, once introduced onto system surfaces, including the electron optical system, are only very slowly removed by the inefficient instrument vacuum pumping system.
Specimens or samples may carry these contaminants into the chamber. These may be part of the specimen, residues from sample preparation techniques or be caused by improper sample handling or storage techniques. For organic specimens, the contamination may be induced by the exposure of the specimen and degradation of the surface by the high energy electron beam. In addition, clean surfaces will accumulate a surface film of hydrocarbon scum if left exposed to ordinary room air for any length of time. The sources of these hydrocarbons are most any living thing, organic object, or other source of hydrocarbon vapors in the vicinity. While the part of the films created in these processes dissipate under vacuum conditions, a small amount generally remains on surfaces and is sufficient to cause problems when the specimen is subsequently examined in the analytical instrument.
Specimens or samples may carry these contaminants into the chamber. These may be part of the specimen, residues from sample preparation techniques or be caused by improper sample handling or storage techniques. For organic specimens, the contamination may be induced by the exposure of the specimen and degradation of the surface by the high energy electron beam. In addition, clean surfaces will accumulate a surface film of hydrocarbon scum if left exposed to ordinary room air for any length of time. The sources of these hydrocarbons are most any living thing, organic object, or other source of hydrocarbon vapors in the vicinity (e.g., solvents and other aromatic compounds). While some of the compounds created in these processes dissipate from surfaces under vacuum and related conditions, other compounds and conditions generally insure residual amounts will remain on surfaces and even minute amounts of this contamination are sufficient to cause problems when the specimen is subsequently examined in the analytical instrument.
Plasma cleaning has been shown to be useful for removing hydrocarbon contamination. A device for cleaning electron microscope stages and specimens is described in U.S. Pat. No. 5,510,624 (Zaluzec) for analytical electron microscopes including TEM. That apparatus uses a plasma generating chamber and an airlock to allow the specimen and stages to be placed into the plasma chamber for cleaning. It may be connected with the analytical chamber of the analytical electron microscope. Several commercial desktop plasma cleaners licensing this patent are sold for cleaning stages and specimen together before inserting into the TEM.
Vane disclosed in U.S. Pat. Nos. 6,105,589, 6,452,315 and 6,610,257 the technology used by XEI Scientific, Inc. in the Evactron® De-Contaminator systems that use an air plasma to produce oxygen radicals for downstream cleaning of electron microscopes and other vacuum systems that have been sold since 1999. These patents describe an oxidative cleaning system and apparatus using low powered RF plasma to produce oxygen radicals, an active neutral species, from air to oxidize and remove these hydrocarbons. The device is mounted on the outside of the electron microscope and the excited gas moves into the specimen chamber by convective flow created by the rough vacuum pump. This device works well on SEMs, but on TEMs has achieved only limited success because of the need to add an additional vacuum pump to get enough flow through the chamber. A recent publication describes this technique using the Evactron® De-Contaminator for SEM with an additional pump (Shin Horiuchi et al., Contamination-Free Transmission Electron Microscopy for High Resolution Carbon Elemental Mapping of Polymers, ACS Nano, 1297 (2009)). As stated by Horiuchi, “[H]owever, the beam-induced specimen contamination in the TEM cannot be reduced by the simple operation of the plasma generator as for SEMs. The reason is assumed that the specimen chamber and vacuum path of the TEM are considerably narrow, where sufficient oxygen radicals cannot be supplied into the chamber simply by the roughing pump of the microscope.”